1. Field of the Invention
The present invention relates to a telecommunications architecture. More particularly, the present invention relates to tandem switching systems for use within a public switched telephone network (PSTN). The present invention enables voice trunking over an asynchronous transfer mode (ATM) network by replacing tandem switches with a distributed virtual tandem switching system that includes a high speed ATM network. The replacement is virtual because as far as the end offices are concerned, the ATM-based distributed virtual tandem switching system is functionally equivalent to the traditional time division multiplexed (TDM) tandem switching system.
2. Background Information
Within the public switched telephone network (PSTN), an originating caller communicates with a destination by establishing a connection between an end office serving the originating caller and an end office serving the destination. FIG. 1 shows the architecture of the current PSTN. In today's PSTN, end office switches 10 are connected to each other via tandem trunk groups 12, direct trunk groups 14, or both tandem trunk groups 12 and direct trunk groups 14. Each trunk within a trunk group is typically a digital service level 0 (DSO) (i.e., 64 kilobits per second) communication line that transmits between the end offices 10 in a time division multiplexed (TDM) manner. When an end office utilizes a direct trunk group 14, the connection between the end offices 10 is without any intermediaries. When an end/central office 10 utilizes a tandem trunk group 12, the connection between end offices 10 is via a tandem switch 16.
The tandem switch or office 16 is an intermediate switch or connection, between an originating telephone call location and the final destination of the call, which passes the call along. Tandem switches are often utilized to handle overflow calls. That is, when all paths are busy on a primary route, e.g., the direct interoffice trunk group 14 between the originating and destination end offices 10, alternative routes through the tandem switch 16 handle the overflow call volume. The tandem switch 16 can also function as a physical path to non-directly-connected offices in addition to functioning as an overflow path for directly connected offices. If the overflow route through the tandem switch 16 becomes full, an alternate final route may be provided. The alternate final route is via another end office 10, thus employing two interoffice trunk groups 14.
Signaling is needed within the PSTN to establish a connection (i.e., setup a telephone call) between a calling party and a destination. The signaling enables line acquisition and sets up call routing, in addition to performing other functions. The signaling can be transmitted through a channel common with the voice data (in-band signaling) or can be transmitted through a dedicated channel (out of band signaling). The dominant signaling protocol currently in use today is transmitted via the dedicated channel and is called Signaling System 7 (SS7).
A conventional connection setup between two end offices 20, 22 in a tandem network is now described with reference to FIGS. 2 and 3. When a calling party 19 (e.g., 235-1111) dials a telephone number (e.g., 676-2222), the originating central office 20 interprets the dialed digits and routes the call to either a direct interoffice trunk group 14 between end offices 20, 22 or a pair of tandem office trunk groups 12 and the corresponding tandem switch 16 between end offices 20, 22. Assuming the pair of tandem office trunk groups 12 and the corresponding tandem switch 16 is utilized, a trunk from each of the trunk groups 12 needs to be selected and reserved by signaling within the SS7 network. Thus, necessary information is transmitted from the originating end office 20 to its associated signaling transfer point 18. Although only a single signaling transfer point is shown in the figures, a network typically includes many signaling transfer points. Thus, each signaling transfer point 18 transfers signals from one signaling link to another signaling link in the SS7 network that transports SS7 messages.
The transmitted information is in the form of an ISUP (ISDN user part) message. It contains a unique point code, which uniquely identifies each end office, corresponding to the originating end office (originating point code (OPC)) and the destination (destination point code (DPC)). Because the message must first go to the tandem office 16, the ISUP message contains the destination point code of the tandem office. The message also contains a circuit identification code (CIC) that corresponds to the physical circuit that will be employed to transport the data. Thus, interoffice trunks are identified by originating point code (OPC), destination point code (DPC), and circuit identification code (CIC).
As shown in the example illustrated in FIG. 3, initially an ISUP message is sent containing a DPC equal to 24612, an OPC equal to 24611, and a CIC equal to 22. Consequently, a circuit will be setup between the originating end office 20 and the tandem office 16. The tandem switch 16 receives the SS7 message and determines from the called number, which is embedded in the protocol, where to route the call, i.e., the appropriate destination end office 22. Then, via the SS7 network, the call is setup between the tandem switch 16 and the appropriate terminating office 22 in a similar manner. Thus, because the tandem office 16 needs to transport the data to the destination end office 22, the tandem office 16 sends an ISUP message to the signaling transfer point 18, including the destination end office=s destination point code, i.e., 24613, the tandem office's origination point code, i.e., 24612, and the circuit identification code corresponding to the circuit between the tandem office 16 and the destination office 20, e.g., circuit 7. After this ISUP message is sent to the signaling transfer point 18, the signaling transfer point 18 forwards the ISUP message to the destination end office 22 in order to setup the connection between the tandem office 16 and the destination office 22, thus reserving the circuit. The terminating central office switch 22 receives the SS7 message and determines where to terminate the call by interpreting the called number embedded in the protocol.
A call flow scenario is now described with reference to FIG. 2. A caller 19 dials the telephone number of a destination 23. The first end office 20 (end office A) collects the digits of the called number and checks routing tables to determine to which end office 22 the dialed telephone number belongs. Then the originating end office 20 finds a direct trunk group 14 between itself and the end office owning the dialed telephone number. Subsequently, the originating end office finds an idle trunk within the trunk group 14. The originating end office 20 selects and reserves the idle trunk of the trunk group 14 and initiates an SS7 IAM (initial address message) message containing the following: signaling transfer point routing address of the destination end office; the calling telephone number; the called telephone number, and the trunk ID (CIC) for the selected trunk of the trunk group.
The signaling transfer point 18 receives the IAM message and forwards it to the destination end office 22. The destination end office 22 then receives the IAM message and uses the CIC information to reserve the selected trunk within the trunk group 14. The destination end office 20 (end office B) then checks the called telephone number 23 for on-hook and feature support and holds the line, assuming the dialed telephone number is on hook. The destination end office 22 then applies a ring to the line and ring tone to the selected trunk in the trunk group 14. Next, the destination end office 22 connects the dialed telephone number line to the selected trunk in the trunk group 14, initiates an SS7 ACM (Address Complete Message) message and forwards it to the signaling transfer point 18.
The signaling transfer point receives the ACM message and forwards it to the originating end office 20 that receives the ACM message. The originating end office 20 then connects the calling telephone number line to the selected trunk. Consequently, the caller of the calling number hears a ring tone and the called party at the called telephone number picks up the phone. The destination end office 22 detects the off hook on the called telephone number 23 and removes the ring tone. The destination end office 22 then initiates an SS7 ANM (answer) message to the signaling transfer point 18. The signaling transfer point 18 receives the ANM message and forwards it to the originating end office 20. The originating end office 20 receives the ANM message and starts necessary billing measurement. Ultimately, the caller speaks with the called party.
Another call flow scenario according to the prior art is now described with reference to FIG. 2. Initially, a caller, e.g., 235-1111 dials a destination, e.g., 676-2222. The originating end office 20 (end office A) collects digits of the called number and checks routing tables to determine which end office handles 676. The originating end office 20 finds that 676 belongs to a destination end office 22 (end office B). End office A then locates a direct trunk group 14 to end office B. Assume in this example that no idle trunk exist within the direct trunk group 14. Thus, end office A chooses and reserves a first tandem trunk group 12, and a selected trunk from the first reserved trunk group 12. Subsequently, end office A initiates an SS7 IAM message containing the following: signaling transfer point routing address of the tandem; calling telephone number; called telephone number; and trunk identification (CIC) for the selected trunk of the first reserved trunk group 12.
The signaling transfer point 18 receives the IAM message and forward it to the tandem switch 16. The tandem office 16 receives the IAM message and utilizes the CIC information to reserve the selected trunk of the first reserved trunk group 12. The tandem office 16 then checks a routing table to determine the destination and reserves a selected trunk of a second trunk group 12, which connects to the destination. Subsequently, the tandem 16 initiates an SS7 IAM message to the signaling transfer point 18 with the following information: signaling transfer point routing address of end office B; calling telephone number; called telephone number; and trunk identification (CIC) for the selected trunk of the second trunk group 12.
The signaling transfer point 18 receives the IAM message and forwards it to end office B. End office B receives the IAM message and utilizes the CIC information to reserve the selected trunk of the second trunk group 12. End office B checks whether the called telephone number is on-hook and holds the line, assuming that 676-2222 is on-hook. End office B applies ringing to the line and a ring tone to the selected trunk of the second trunk group 12. End office B then connects the line to the selected trunk of the second trunk group 12 and initiates an SS7 ACM message to the signaling transfer point 18.
The signaling transfer point 18 receives the ACM message and forward it to the tandem switch 16. The tandem switch 16 receives the ACM message from the signaling transfer point 18 and consequently, the tandem switch initiates an ACM message to the signaling transfer point 18.
The signaling transfer point 18 receives the ACM message and forwards it to end office A. End office A receives the ACM message and connects 235-1111 to the selected trunk of the first reserved trunk group 12. Next, the caller at 235-1111 hears a ring tone and the called party at 676-2222 picks up the phone.
Consequently, end office B detects an off-hook on 676-2222. Accordingly, end office B removes the ring tone and initiates an ANM message to the signaling transfer point 18. The signaling transfer point 18 receives the ANM message and forwards it to the tandem switch 16. The tandem switch 16 receives the ANM message from the signaling transfer point 18 and the tandem switch 16 initiates an ANM message to the signaling transfer point 18.
The signaling transfer point 18 receives the ANM message from the tandem switch and forwards it to end office A. End office A receives the ANM message from the signaling transfer point 18 and starts necessary billing measurement. Finally, the calling party at 235-1111 talks to the called party at 676-2222.
The current system has disadvantages. In order to minimize overflow call volume, the size of a trunk group needs to be forecast so that the trunk group can handle the expected call volume. Then, appropriately sized trunk groups are preprovisioned, each having a dedicated bandwidth. The process of forecasting and preprovisioning is expensive. Moreover, the current trunking architecture requires a large number of small trunk groups to link end offices because of the large number of end offices that each end office must connect with. This form of trunking leads to inefficiencies due to the relatively small size of a significant number of the trunk groups. That is, the small size reduces the call carrying capacity per trunk and therefore requires a larger percentage of overflow trunking. In addition, the large number of trunk groups requires huge investments in hardware and software for systems that keep track of individual interoffice trunks. Further, the trunk forecasting and provisioning is necessary for thousands of individual trunk groups.
The ATM Forum's VTOA Group has attempted to solve the problems associated with voice trunking over ATM. The VTOA Group developed a specification for carrying voice over ATM in a private network environment. For example, see ATM Forum Technical Committee, "Circuit Emulation Service Interoperability Specification Version 2.0" (January 1997). That specification allows private businesses to employ an ATM network to establish voice channels across the ATM network using a protocol, such as private network-network interface (PNNI), which facilitates moving cells from one point in the ATM network to another point in the ATM network. However, the specification is limited to application within a private environment, which is not appropriate for applications in the PSTN. That is, interaction is not supported with systems that include out-of-band signaling, e.g., Signaling System 7 (SS7), which is essential to supporting capabilities such as an advanced intelligent network (AIN).
Within these private networks, the signaling is typically in-band signaling. Thus, no interface with an out-of-band signaling network would be required. Moreover, if a calling party within the private network would like to contact someone outside of the private network, the calling party must communicate over the normal PSTN, thus leaving the scope of the VTOA Group's system.
U.S. Pat. No. 5,483,527 addresses voice trunking within the PSTN. The patent discloses a system that interposes an ATM network between two central offices. Signaling is sent from the central office via a signaling transfer point (STP) to the ATM switch. Within each ATM switch, a processing system is provided for interfacing the ATM switch with the STP. Thus, the ATM switches are modified to be able to communicate with the signaling transfer point, which is a very expensive process. Furthermore, due to the interface being provided within each ATM switch, the path across the ATM network is established relatively slowly. Finally, the distributed placement of the interface between the signaling transfer points and the ATM network has its own disadvantages.
Glossary of Acronyms AAL ATM Adaptation Layer ACM Address Complete Message ADPCM Adaptive Differential Pulse Code Modulation ADSL Asymmetric Digital Subscriber Line AIN Advanced Intelligent Network ANM Answer Message ANSI American National Standards Institute ATM Asynchronous Transfer Mode B-ISUP Broadband ISDN User Part CAS Channel Associated Signaling CBR Constant Bit Rate CCS Common Channel Signaling CES Circuit Emulation Service CIC Circuit Identification Code CS-IWF Control and Signaling Interworking Function DPC Destination Point Code DS0 Digital Signal Level 0 (64 Kbps digital signal format) DS1 Digital Signal Level 1 (1.544 Mbps digital signal format) IAM Initial Address Message IP Internet Protocol ISDN Integrated Service Digital Network ISUP ISDN User Part ITU-T International Telecommunications Union - Telecommunications IWF Interworking Function IXC Interexchange Carrier OAM&P Operations, Administration, Maintenance, and Provisioning OC12 Optical Carrier level 12 signal (622 Mbps) OC3 Optical Carrier level 3 signal (155 Mbps) OPC Originating Point Code PCM Pulse Code Modulation PNNI Private Network-Network Interface POTS Plain Old Telephone Service PSTN Public Switched Telephone Network SS7 Signaling System 7 SSP Service Switching Point STP Signal Transfer Point SVC Switched Virtual Connection TDM Time Division Multiplexing T-IWF Trunk Interworking Function UNI User-to-Network Interface VTOA Voice and Telephony over ATM